The leader of the Dignity Party, Hamdeen Sabahi, also pledged to participate, but said the opposition wanted assurances that the election will be conducted fairly.

Tensions in Egypt have run high since President Mohamed Morsy granted himself expanded presidential powers last month and an Islamist-dominated council rushed through a draft constitution that opposition groups say fails to adequately protect the rights of Egypt's religious minorities and women, among other issues.

The outcome of the election, and the unrest associated with it, are important to the stability of volatile North Africa and the Middle East -- where Egypt is a key player -- and the situation is being watched closely around the world.

Until Wednesday, the day when voting opened for Egyptians living abroad, it was not entirely clear how the major opposition parties would approach the referendum.

The National Salvation Front, a coalition of liberal and secular opposition parties, urged its supporters to participate if it's not able to get the vote put off for a month. Sabahi said the opposition wants close judicial and nongovernmental monitoring of the vote, a one-day referendum and immediate and detailed announcement of the results.

The Strong Egypt party and the Egyptian Current party, both founded by former Muslim Brotherhood members, as well as the 6 April youth movement, which backed Morsy in the June elections, had previously decided to participate and urged members to vote no, according to the semiofficial Al-Ahram newspaper.

"We are organizing campaigns all over the country against the draft constitution, trying to explain to the people why we think that this is not the constitution of the revolution," Al-Ahram quoted 6 April founder Mohamed Adel as saying.

While many parties now say they will participate in the vote, Egypt's military said Wednesday that there was little interest in a meeting it had set up to foster dialogue among the nation's myriad political movements. The Defense Ministry canceled the meeting, scheduled for Wednesday, after many of those invited declined to attend.

The meeting might be held later, the ministry said.

Meanwhile, Human Rights Watch called on Egyptian prosecutors to look into the detention and treatment of protesters who the group said were detained and in some cases beaten December 5 by Muslim Brotherhood members.

"Unlawful detention and abuse occurring at the gates of the presidential palace, and in the presence of the riot police, raises real concerns that the presidential palace was aware of these abuses and did nothing to stop them," Human Rights Watch said Wednesday in a statement.